1. Field of the Invention
The present invention relates to a display device, and more particularly, to a dual panel type organic electroluminescent (EL) display device and a method of fabricating the same.
2. Discussion of the Related Art
Among flat panel displays (FPDs), organic electroluminescent (EL) display devices have been of particular interests in research and development because they are light-emitting type displays having superior brightness, wide viewing angle and high contrast ratio. In particular, an organic EL display device is a self-luminous device and does not need an additional light source to emit light. Accordingly, an organic EL display device has a very thin profile and light weight.
In addition, the organic EL display device can be operated using a low direct-current (DC) voltage, thereby having low power consumption and fast response time. Further, the organic EL display device is an integrated device, such that it has high endurance of external impacts, a large operational temperature range and a wide range of applications. Moreover, the organic EL display device generally is manufactured using a relatively simple process including a deposition process and an encapsulation process. Thus, an organic EL display device has a low production cost.
An active matrix type organic EL display device includes thin film transistors as switching elements within each pixel. The voltage applied to the pixels are charged in a storage capacitor Cst so that the voltage can be applied until the next frame signal is applied, thereby continuously driving the organic EL display device regardless of the number of gate lines until a picture of images is finished. Accordingly, the active matrix type organic EL display device provides uniform luminescence, even when a low current is applied and the display area is large.
FIG. 1 is a schematic cross-sectional view of an organic EL display device according to the related art. In FIG. 1, an organic EL display device includes first and second substrates 10 and 60 facing each other and spaced apart from each other. An array element layer AL is formed on the first substrate 10 and includes a thin film transistor (TFT) T. Although not shown, the array element layer AL further includes a gate line, a data line crossing the gate line to define a pixel region P, and a power line crossing one of the gate and data lines. In addition, a first electrode 48, an organic electroluminescent (EL) layer 54 and a second electrode 56, which constitute an organic EL diode DEL, are sequentially formed on the array element layer AL. The first electrode 48 is connected to the TFT T.
In addition, the second substrate 60 functions as an encapsulating panel having a receded portion 62. A desiccant 64 is packaged in the receded portion 62 to protect the organic EL display device from moisture. A seal pattern 70 is formed between the first and second substrates 10 and 60 in a periphery thereof. With the seal pattern 70, the first and second substrates 10 and 60 are attached to each other.
Accordingly, the organic EL display device according to the related art emits light from the organic EL diode DEL toward the array element layer AL and away from the second substrate 60.
FIG. 2A is a schematic plan view of a pixel region of the organic EL display device shown in FIG. 1. As shown in FIG. 2A, a gate line 22 crosses a data line 42 and a power line 28, and the data line 42 and the power line 28 are spaced apart from each other. The pixel region P is defined by the gate line 22 and the data line 42. A switching TFT TS is located adjacent to the crossing of the gate line 22 and the data line 42. A driving TFT Td is connected to the switching TFT TS and the power line 28. A storage capacitor CST uses a portion of the power line 28 as a first capacitor electrode and an active pattern 16 extending from an active layer 31 of the switching TFT TS as a second capacitor electrode. The first electrode 48 is connected to the driving TFT Td. The switching TFT TS and the driving TFT Td constitute a TFT T. Although not shown, the organic EL layer 54 and the second electrode 56 (shown in FIG. 1) are sequentially formed on the first electrode 48.
FIG. 2B is a schematic cross-sectional view along II-II of FIG. 2A. As shown in FIG. 2B, the driving TFT Td including an active layer 14, a gate electrode 20, source electrode 38, and drain electrode 40 is formed on the first substrate 10. The source electrode 38 connects to the power line 28 through a power electrode 26 that is connected to the power line 28, and the drain electrode 40 connects to the first electrode 48. The active pattern 16 is formed with the same material as the active layer 14 and is formed under the power line 28 having conductivity. The active pattern 16 and the power line 28 constitute the storage capacitor CST. The organic EL layer 54 and the second electrode layer 56 are sequentially formed on the first electrode 48. The first electrode 48, the organic EL layer 54, and the second electrode 56 constitute the organic EL diode DEL.
In addition, a first insulating layer 12 is formed between the first substrate 10 and the active layer 14 as a buffer layer. A second insulating layer 18 is formed between the active layer 14 and the gate electrode 20 as a gate insulating layer. A third insulating layer 24 is formed between the active pattern 16 and the power line 28. A fourth insulating layer 30 is formed between the power line 28 and the source electrode 38. A fifth insulating layer 44 is formed between the drain electrode 40 and the first electrode 48. A sixth insulating layer 50 is formed between the first electrode 48 and the second electrode 56. The third to sixth insulating layers 24, 30, 44 and 50 include contact holes for electric connections of the respective electrodes.
In the organic EL display device according to the related art, the array element layer having TFTs and the organic electroluminescent (EL) diode are formed on the first substrate, and the second substrate is attached to the first substrate for encapsulation. However, when the array element layer having TFTs and the organic EL diode are formed on one substrate, production yield of the organic EL display device is determined by a multiplication of the array element layer's yield and the organic EL diode's yield. In particular, because the yield of the organic EL diode is relatively low, the production yield of the overall EL display device is limited by the yield of the organic EL diode. For example, even when TFTs are well fabricated, an organic EL display device using a thin film of about 1000 Å thickness can be determined to be defective due to a defect of an organic emission layer. This results in loss of materials and high production costs.
In addition, organic EL display devices are classified into bottom emission type and a top emission type based on a direction of light emitted from the organic EL diodes. The bottom emission type organic EL display devices have advantages such as high encapsulation stability and high process flexibility. However, the bottom emission type organic EL display devices are ineffective for high resolution devices because they have a low aperture ratio.
In contrast, the top emission organic EL display devices have a higher expected life span because they are more easily designed and have a high aperture ratio. However, in the top emission type organic EL display devices, the cathode is generally formed on an organic emission layer. As a result, transmittance and optical efficiency of the top emission type organic EL display devices are reduced because of a limited number of materials that can be selected. Further, when a thin film passivation layer is formed to avoid the reduction of light transmittance, the thin film passivation layer may fail to block infiltration of exterior air into the device.